1. Field of the Invention
This invention relates to a semiconductor optical amplification element for use with optical communication, and more particularly to a semiconductor optical amplification element which has an optical signal amplification function or an optical signal quenching function.
2. Description of the Related Art
A semiconductor optical amplification element (hereinafter referred to as optical amplifier) is an element which has an optical amplification medium of the striped optical waveguide type and amplifies an optical signal without electrically converting it. While the optical amplifier has a basic structure common to that of a semiconductor laser, an anti-reflecting coating or the like is applied to each end of the element to suppress the light reflection factor at the end so that the optical amplifier may have no optical resonator structure. By such suppression of the light reflection factor, laser oscillation of the optical amplifier itself when carriers are injected is suppressed and amplified light is outputted by stimulated emission by input signal light.
Since the reflection factor at each end of the element is usually required to be lower than 0.1%, in addition to an anti-reflecting coating applied to each end of the element, a structure which prevents reflected light from each end facet of the element from being recoupled to the optical waveguide is adopted in order to effectively lower the reflection factor at the end of the element.
One of methods of lowering the effective reflection factor at each end of the element is to provide a window region for allowing the optical waveguide structure to be terminated in the proximity of the end facet of the element. Several structures which involve provision of such a window region as described above have been reported, and, for example, the following documents (1) to (3) disclose such structures:
(1) Japanese Patent Laid-Open Application No. Showa 61-96787
(2) P. Doussiere et al., J. Photonicis Technol. Lett., PTL-6, pp.170-172 (1994)
(3) I. Cha et al., Electr. Lett., Vol. 25, pp.1242-1242 (1989)
Also another method wherein the direction of stripes of the optical waveguide is inclined from the direction of a normal to an end facet of the element is known and disclosed, for example, in the following documents (4) and (5):
(4) C. E. Zah et al., Electr. Lett., Vol. 23, pp.990-991 (1987)
(5) Japanese Patent Laid-Open Application No. Heisei 3-136388
In recent years, several further methods which adopt both of the structures of a window region and inclined stripes have been proposed and are disclosed, for example, in the following documents (6) to (8):
(6) L. F. Tiemeijer, OAA '94 Technical Digest, 34/WD1-1 (1994)
(7) P. Doussiere et al., ECOC '96, Proceeding, Vol. 3, WeD.2.4 (1996)
(8) S. Chelles et al., ECOC '96, Proceeding, Vol. 4, ThB.2.5 (1996)
By such various measures as described above, under the preset conditions, an element gain of 30 dB and an amplified signal light saturation output of approximately 10 dBm are realized. While residual light reflection at an end of the element produces gain oscillations (ripple) by wavelength dependency at distances of several angstrom, also such ripple is commonly suppressed lower than 1 dB. Further, an S/N ratio deterioration amount of 5 to 7 dB by noise light carried on a signal upon optical amplification is obtained. The S/N ratio deterioration amount is called "noise figure" and desired to be as small as possible and is determined by an element structure and a current injection amount.
One of devices competent with an optical amplifier of a semiconductor is an optical amplifier in the form of a rare earth element doped fiber (fiber optical amplifier). A fiber optical amplifier having a gain higher than 30 dB, a saturation output higher than 15 dBm and a noise figure of approximately 4 dB has been reported.
While an optical amplifier of a semiconductor is less advantageous in gain characteristic than a fiber optical amplifier as described above, it is advantageous in that it is compact, less expensive and low in power consumption and allows integration. However, further output increase in output and reduction in noise are expected.
With an optical amplifier, although laser oscillation is suppressed by reduction in reflection factor at each end facet of the element, amplified spontaneous emission light (ASE light) is emitted. The amplified spontaneous emission light makes noise light, and in order to reduce noise light to the outputting side, reduction of the amplified spontaneous emission light is desired.
Further, S/N ratio deterioration of signal light is caused also by gain saturation in an optical amplifier. Therefore, improvement in saturation output is desired.